This invention relates to a piston for internal combustion engines preferably made of aluminum or its alloys and in which the freely suspended piston pin bosses are connected by respective ribs to the piston head and the piston skirt and the piston skirt is formed with apertures in register with the bores in the piston pin bosses.
Otto-cycle and Diesel type engines in which one-metal pistons made preferably of aluminum or its alloys, must meet particularly high requirements as regards silent running, oil consumption, seizure-proofness, groove wear and general adaptability to greatly changing operating conditions, and particularly as regards the straight-line motion of the piston. It is conventional to use so-called controlled-expansion pistons for such engines.
In the broadest sense, the term controlled-expansion pistons is applicable to pistons which embody special structural features that ensure that the thermal expansion in a radial direction takes place preferentially only in the direction of the boss axis or in which the carrying skirt portions are designed so that they can elastically yield to take up a thermal expansion in a radial direction.
In a narrow sense the term is applicable to pistons which comprise expansion-resisting control members of steel that are embedded in the piston casting at suitable locations, in most cases in the skirt, and prevent an excessively large expansion of the light metal at operating temperature. This expansion is particularly undesired at right angles to the boss axes. These expansion control members divert said expansion in the direction of the boss axis and ensure that the clearance of the piston in the pressure and back-pressure directions is adapted to the cylinder diameter under all operating conditions as far as possible. In this way the nominal clearance of the piston, measured in its direction of movement, may be held almost constant throughout the range of operating conditions although the cylinder expands to a larger diameter at elevated temperature. The expansion control action is equivalent to the coefficient of the thermal expansion of the piston and is determined by the temperature gradient between the piston head and the skirt. This gradient is different for each engine and is affected by design features.
The coaction of the clearance of the piston, the expansion control action and the deformation of the skirt is decisive for the straight-line motion of the piston. It must always be borne in mind that a compromise must be found between the requirements for an adequate stiffness, stress concentration factor and expansion control action. Only with such compromise will the requirements as regards silent running and seizure-proofness be fulfilled.
The operating temperature results in a mean coefficient of the thermal expansion, which lies between the coefficients of thermal expansion of the light alloy and steel, depending on the ratio of the wall thicknesses. In view of the temperature gradient between the piston head and skirt, this mean coefficient of thermal expansion results in an increase of the differential deformation over that which is due to the temperature difference so that the action of the piston head to deform the skirt to an oval shape is increased further. Because the circumference of the skirt is substantially constant, the diameter is increased in the direction of the pins and is decreased in the pressure and back-pressure directions.
Because the expansion of the light metal is not entirely prevented, the ovality of the skirt must be larger than normal. Almost all controlled-expansion pistons exhibit a more or less pronounced separation between the piston head and skirt. For instance, the expansion control action is greatly promoted by separating slots, which lie preferably in the oil scraper ring groove. Pistons which have steel inserts but lack an appreciable separation between the head and skirt exhibit only a small expansion control action.
The known types of controlled-expansion pistons (German Pat. No. 909,163, Printed German Application No. 1,078,387, German patent specification No. 1,245,657), have an expansion control action of 15-20.times.10.sup.-6 mm/mm.degree.C. at the upper rim of the skirt and of 18-22.times.10.sup.-6 mm/mm.degree.C. at the lower rim of the skirt and are restricted in application to specific ranges as regards horsepower per unit of displacement and as regards piston diameter.
In a sense, so-called slotted-skirt pistons are also controlled-expansion pistons. These pistons, which lack cast-in expansion control members of steel, were mainly used in the forties and are still used today in some cases. A feature resides in that the skirt is separated from the hot piston head by two slots in or under the lower-most groove and one or more longitudinal slots are provided in the skirt at suitable locations.
The transverse slots provide for a certain expansion control action and the longitudinal slots impart a high elasticity to the skirt. The increase of the light metal in diameter during operation is taken up by the elastic deformation of the skirt. Because the skirt is yieldable and adaptable, slotted-skirt pistons are highly seizure-proof. On the other hand, they have the serious disadvantage that they lack a sufficiently high creep strength of the light metal at elevated temperatures and at the high frequency of load reversals which is due to higher engine speeds and that permanent deformations, resulting in a higher noise, possibly in seizure and in an incipient and complete fracture of the skirt must be expected after a prolonged operating time.